Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity, produce polymers having medium to broad molecular weight distributions (M.sub.w /M.sub.n &gt;4), and are generally poor at incorporating .alpha.-olefin comonomers. To improve polymer properties, highly active single-site catalysts, in particular metallocenes, are beginning to replace Ziegler-Natta catalysts. Although more expensive, the new catalysts give polymers with narrow molecular weight distributions, and good comonomer incorporation, which allows easier production of low-density polymers. One disadvantage of metallocene catalysts is that they tend to produce lower molecular weight polymers at higher temperatures.
Recent attention has focused on developing "constrained geometry" or "open architecture" single-site catalysts. These catalysts, which contain a transition metal bound to a multidentate ligand, are believed to have exposed active sites that impart unique properties to the catalyst. U.S. Pat. Nos. 5,703,257, 5,347,024, 5,096,867, and 5,064,802 disclose constrained geometry catalysts having ligands comprising a substituted or unsubstituted cyclopentadienyl ring that is covalently linked to a divalent ligand group comprising nitrogen, phosphorus, oxygen, or sulfur. The cyclopentadienyl group is .pi.-bonded to the metal and the divalent ligand group is .sigma.-bonded to the transition metal. U.S. Pat. Nos. 5,541,349 and 5,495,036 disclose similar constrained geometry catalysts in which the cyclopentadienyl ring is replaced by non-aromatic dienyl ligands. Further, U.S. Pat. No. 5,688,880 discloses constrained geometry catalysts in which the cyclopentadienyl ring is replaced by a delocalized, .pi.-bonded group.
However, heteroatom-containing .pi.-bonded groups are not disclosed and, in particular, indolyl systems are not disclosed. Indolyl catalyst systems for olefin polymerization are disclosed in U.S. Pat. No. 5,539,124, but the indolyl group is not covalently linked to a divalent ligand group. PCT Intl. Appl. WO 96/13529 discloses monoanionic multidentate ligands having a heteroatom-containing cyclopentadienyl ring substituent and a trivalent amine or phosphine substituent, but dianionic ligands are not disclosed.
Constrained geometry catalysts are believed to have unique properties. U.S. Pat. No. 5,278,272 discloses that ethylene/1-octene copolymers produced by a constrained geometry catalyst have much better processability and higher melt elasticity than similar polymers produced by metallocene catalysts. One significant problem with constrained geometry catalyst systems is that they are difficult to prepare, requiring complicated multi-step synthetic pathways. For example, see Examples 1-3 in U.S. Pat. No. 5,688,880.
A cost-effective route to single-site catalysts starts with readily available organic compounds that can act as stable ligands for transition metals. For example, U.S. Pat. No. 5,637,660 discloses single-site catalysts in which a cyclopentadienyl moiety of a metallocene is replaced by a quinolinyl or pyridinyl ligand. Such a route has not been attempted for constrained geometry single-site catalysts.
In sum, new constrained geometry single-site catalysts are needed. Particularly valuable catalysts would be easily synthesized from readily available starting materials. These catalysts would combine the cost advantages of Zeigler-Natta catalysts with the polymer property advantages of constrained geometry catalysts.